Q. Find the average value of the function f(x)=9−4x6 from x=3 to x=9. Express your answer as a constant times ln3.Answer: □ln3
Calculate Interval Width: To find the average value of a continuous function f(x) on the interval [a,b], we use the formula for the average value of a function on an interval, which is given by:Average value = (b−a)1∫abf(x)dxHere, a=3 and b=9, and f(x)=(9−4x)6.
Set Up Integral: First, we calculate b−a, which is the width of the interval.b−a=9−3=6
Perform u-Substitution: Next, we set up the integral to find the average value.Average value = (1/6)×∫399−4x6dx
Rewrite Integral in u: To integrate f(x)=9−4x6, we perform a u-substitution.Let u=9−4x, then du=−4dx, or dx=−4du.When x=3, u=9−4(3)=9−12=−3.When x=9, u=9−4(9)=9−36=−27.
Simplify Integral: Now we rewrite the integral in terms of u.Average value = 61 * ∫−3−27u6∗(−41)du
Evaluate Integral: We can simplify the integral by taking constants out.Average value = (61)×(4−6)×∫−3−27(u1)duAverage value = (−41)×∫−3−27(u1)du
Remove Absolute Value: The integral of u1 du is ln∣u∣.Average value = 4−1 * (ln∣−27∣ - ln∣−3∣)
Apply Logarithm Property: Since the natural logarithm function ln∣x∣ is the same for positive and negative values of x, we can remove the absolute value signs.Average value = (−1/4)×(ln(27)−ln(3))
Simplify Natural Log: We use the property of logarithms that ln(a)−ln(b)=ln(ba).Average value = (−41)⋅ln(327)Average value = (−41)⋅ln(9)
Correct Limits of Integration: We know that ln(9) is the same as 2×ln(3).Average value = (−41)×2×ln(3)Average value = (−21)×ln(3)
Correct Limits of Integration: We know that ln(9) is the same as 2×ln(3). Average value = (−1/4)×2×ln(3) Average value = (−1/2)×ln(3)Correcting the limits of integration after the u-substitution, we should have: Average value = (1/6)×∫−27−3(6)/u×(−1/4)du This changes the sign of the integral because the limits are now in the correct order. Average value = (1/6)×(6/4)×∫−27−3(1/u)du Average value = (1/4)×(ln∣−3∣−ln∣−27∣)
Correct Limits of Integration: We know that ln(9) is the same as 2×ln(3). Average value = (−1/4)×2×ln(3) Average value = (−1/2)×ln(3)Correcting the limits of integration after the u-substitution, we should have: Average value = (1/6)×∫−27−3(6)/u×(−1/4)du This changes the sign of the integral because the limits are now in the correct order. Average value = (1/6)×(6/4)×∫−27−3(1/u)du Average value = (1/4)×(ln∣−3∣−ln∣−27∣)Removing the absolute value signs and using the property of logarithms: Average value = (1/4)×(ln(3)−ln(27)) Average value = (1/4)×ln(3/27) Average value = (1/4)×ln(1/9)
Correct Limits of Integration: We know that ln(9) is the same as 2×ln(3). Average value = (−1/4)×2×ln(3) Average value = (−1/2)×ln(3)Correcting the limits of integration after the u-substitution, we should have: Average value = (1/6)×∫−27−3(6)/u×(−1/4)du This changes the sign of the integral because the limits are now in the correct order. Average value = (1/6)×(6/4)×∫−27−3(1/u)du Average value = (1/4)×(ln∣−3∣−ln∣−27∣)Removing the absolute value signs and using the property of logarithms: Average value = (1/4)×(ln(3)−ln(27)) Average value = (1/4)×ln(3/27) Average value = (1/4)×ln(1/9)Since 2×ln(3)0 is the same as 2×ln(3)1, we have: Average value = 2×ln(3)2 Average value = (−1/2)×ln(3) However, we need to correct the sign due to the earlier mistake. The average value should be positive, so we have: Average value = 2×ln(3)4